1. Field of the Invention
The present invention relates to a method and apparatus for controlling the molecular structure or melt flow rate of the polypropylene produced from a reactor in a polypropylene slurry process and for controlling the reactor slurry concentration to maximize the capacity of the process and reduce the solvent recovery cost.
Two important variables in the polypropylene (PP) slurry process are melt flow rate (MFR) and reactor slurry concentration (SCON). It is very difficult, if not impossible, to measure MFR and SCON directly in a slurry reactor. Therefore, it is difficult to control the process variables using conventional control strategies. Presently, MFR and SCON are controlled by manually adjusting process variables that can be measured directly.
As will be described in greater detail hereinafter, the advanced control strategies of the present invention utilize an on-line process simulation program to monitor and control MFR and SCON and by improving MFR and SCON control, improved product quality, maximized capacity and reduced solvent recovery costs, are obtained.
2. Description of the Prior Art
Heretofore various control strategies for controlling the melt flow rate and reactor slurry concentration in a polypropylene slurry process have been proposed. Examples of such prior control strategies are set forth in the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,087,917 Scoggin 3,250,757 Smith et al 3,257,363 Miller et al 3,356,667 Smith et al 3,476,729 Smith et al 3,492,283 Miller 3,551,403 Delbouille et al 3,614,682 Smith 3,951,604 Smith et al 3,998,995 Buss et al 4,469,853 Mori ______________________________________
The Scoggin U.S. Pat. No. 3,087,917 discloses a polymerization process which uses a diluent and produces a solid polymer in particle form suspended in a slurry in a polymerization vessel. The slurry is maintained at a desired temperature by taking a portion of the diluent separated from the slurry, cooling the diluent portion, returning it to the vessel and controlling the temperature of the cooled, reintroduced diluent.
The Smith et al. U.S. Pat. No. 3,250,757 discloses a method and apparatus for controlling feed to a polymerization reactor. According to the method disclosed, one or more of several feed streams, including recycle monomer stream, make up monomer stream, the combined monomer stream, and the diluent make-up stream, are controlled to maintain a substantially constant monomer concentration and total flow rate for the combined streams when employing a substantial recycle stream of unreacted monomer and diluent.
The Miller et al. U.S. Pat. No. 3,257,363 discloses a process for polymerization of 1-olefin reactant in a liquid phase in a reaction zone in the presence of a liquid hydrocarbon vehicle wherein a polymer product is formed the density of which is different than the density of the non-product phase or the resulting reaction mixture, and the flow rate of said vehicle passed to said zone is manipulated in response to the computed concentration of the product in the reaction mixture to maintain a predetermined concentration of the product in the reaction mixture. The improvement disclosed in this patent comprises measuring the density of the reaction mixture and producing a signal proportional thereto, analyzing the reaction mixture to determine the concentrations of the reactant and vehicle in the reaction mixture and producing a signal proportional thereto, and controlling the flow rate of the vehicle to the reaction zone in response to the latter signal to maintain a predetermined concentration of the product in the reaction mixture.
The Smith et al. U.S. Pat. No. 3,356,667 discloses control of polypropylene polymer quality by establishing a first output signal representative of hydrogen concentration in the feed to the polymerization zone, establishing a second output signal representative of the percent solids (polymer solids) in the reaction zone, establishing a third output signal representative of the rate of flow of monomer to the polymerization reaction zone, establishing a fourth output signal representative of the temperature of the reaction mixture within the reaction zone, producing a first control output signal representative of the instantaneous polymer melt index in the reaction mixture in the reaction zone, and varying the rate of hydrogen addition to the reaction zone in response to changes in the thus-determined polymer melt index (instantaneous) which is equivalent to the MFR.
The Smith et al. U.S. Pat. No. 3,476,729 discloses a polymerization process control and apparatus therefor. The amount of modifier added to a polymerizable monomer stream is controlled by sampling the concentration of the modifier in the monomer stream, generating a signal representative of this concentration, and regulating the flow of the added monomer in response to this signal so as to maintain the concentration of the modifier in the monomer stream at a predetermined level. The regulation of the modifier flow is maintained independent of any pressure fluctuations in the polymerization system.
The Miller U.S. Pat. No. 3,494,283 discloses a method for control of chemical reactions. In a polymerization system, the actual density of the reaction mixture is measured and the concentration of the polymer in the reaction mixture is determined. In one embodiment, a signal representative of the density which the reaction mixture would have with the determined polymer concentration and the desired monomer concentration is established and compared with the actual density to obtain a control signal for the manipulation of the monomer flow rate to the reactor to maintain the actual monomer concentration substantially at the desired value therefor. In a second embodiment, the actual monomer concentration is determined and compared with the desired monomer concentration to obtain the control signal. The desired monomer concentration and desired reaction temperature can be established responsive to the desired values of melt flow rate and melt index of the polymer to be produced.
The Delbouille et al. U.S. Pat. No. 3,551,403 discloses a process for the regulation of the polymerization of gaseous olefins in a reactor operating continuously at a low pressure and in the presence of a diluent, a polymerization catalyst and a chain transfer agent. The process which is carried out under constant condition of temperature and pressure and with substantially constant concentrations is regulated by immediately adjusting the olefin input flow into the reactor in response to a variation in the gaseous output from the process and then adjusting the input flow of catalyst in the reactor to return the input flow of olefin to its initial value. An apparatus for carrying out the method includes a continuous type reactor, a separator communicating with the reactor, means for maintaining constant conditions in the reactor and separator and means for adjusting the concentrations of olefin and catalyst in the reactor in response to the gaseous outflow from the separator.
The Smith U.S. Pat. No. 3,614,682 discloses digital computer control of a polymerization process by determining the value of a process variable, generating and storing digital signals corresponding to numerical values of the process variables, storing a program for controlling the process, and periodically executing the program.
The Smith et al. U.S. Pat. No. 3,951,604 discloses a polymerization system having first and second feed streams, diluent and recycled diluent streams and a modifier stream controllably fed to a reactor, a resultant product stream is passed from the reactor and a gas stream is removed from the resultant bottoms product of the reactor. The gas stream is analyzed and signals are delivered in response to the analysis. The signals are modified and compared to a modified signal responsive to one of the flow rates of the recycled diluent stream or the second feed stream that has been modified by the flow rate of the diluent stream for providing a resultant control signal for relatively controlling the flow of the modifier stream.
The Buss et al. U.S. Pat. No. 3,998,995 describes polyethylene polymerization in a "tubular reactor" in which the monomer flow to the reaction zone is controlled to maintain a given polymer production rate. This patent teaches the determining of polymer formation rate by making a heat balance around the reactor. A control system determines the major monomer feed rate required to yield a desired polymer production rate. The major monomer feed rate is then set by and adjusted in response to this determination of monomer feed rate to maintain the heat balance determined production rate unless the major monomer concentration in the reactor increases beyond a predetermined high limit.
In contradistinction, the method and apparatus of the present invention, relate to a process to control the polymer slurry concentration in a stirred tank reactor for the polymerization of propylene by manipulating the diluent (hexane) feed to the reactor. The production rate and the monomer feed rate can vary, yet the slurry concentration adjusting the diluent feed to compensate for changes in production rate or monomer feed rate.
The Mori U.S. Pat. No. 4,469,853 describes a method to produce a polyolefin of a predetermined melt index (for polypropylene melt index is also known as melt flow rate) by maintaining the reactor temperature constant, detecting by gas chromatography the concentrations of the olefin and hydrogen in the gas phase and controlling the concentrations of olefin and hydrogen to produce a polyolefin of a given melt index.
In contradistinction, the method and apparatus of the present invention detect the concentrations of propylene and hydrogen and control the ratio of hydrogen concentration to propylene concentration at the measured temperature and pressure of the reactor in order to maintain the ratio at a desired value to control the melt flow of the polymer being produced. The reactor temperature or pressure can change since they are controlled by separate control loops, but the ratio of hydrogen to propylene is changed based on the measured reactor temperature and pressure to produce a given MFR polymer product.